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The present invention relates to radio frequency traps and in particular to a floating trap suitable for use with magnetic resonance imaging equipment.
Electrical conductors used for transmitting signals susceptible to external electromagnetic noise often employ a center conductor surrounded by a conductive shield. The shield is grounded to prevent external electric fields from influencing the signal on the central conductor. A common xe2x80x9ccoaxial cablexe2x80x9d shielded conductor, used for radio-frequency signals, employs a braided shield surrounding a central multi-strand conductor separated from the braided shield by an insulator of predetermined diameter and dielectric properties. The braded shield is surrounded in turn by a second insulator that protects the shield from damage or electrical contact with other conductors.
In applications where there are intense external electrical/magnetic fields, for example, in magnetic resonance imaging (MRI), significant current may be induced in the shield causing failure of the shielding effect and possibly damage to the shield and its adjacent insulation from heating. One method of reducing shield current is with ferrite xe2x80x9cbeadsxe2x80x9d which fit over the shield to resistively damp eddy currents induced by the shield currents and thus the shield currents themselves. It is also known to reduce such shield currents by creating an S-trap in which the coaxial cable is wound in a first direction and then optionally a second direction about a cylindrical form to produce a self-inductance among the coils of each winding set. A capacitance is connected in parallel with the inductance (by attaching leads of a capacitor to the shield at separated points in each winding) providing parallel resonant circuits tuned to the particular frequency of the offending external radio frequency field. The resonance provides the shield with a high impedance at the frequency of the interference, resisting current flow at this frequency, while the counter-winding reduces inductive coupling of the trap to the noise.
While the S-trap may successfully reduce current flow in the shield, it requires additional cable length for the windings and thus may contribute to a loss of signal strength and may introduce an undesirable phase change in the signal. Further, manufacture of the S-trap is cumbersome, requiring modification of the coaxial cable, including a removal of portions of its external insulation for attachment of a capacitor. The fixed position of the S-trap makes it difficult to adjust the S-trap to a location on the shield having maximum current, as is desirable. Ferrite beads are unsuitable in areas of intense magnetic fields, such as are found in magnetic resonance imaging machines.
Co-pending U.S. application Ser. No. 10/145,229 filed May 13, 2002 describes a shield current trap having a first and second, concentric, tubular conductor electrically connected to provide a resonance-induced high impedance to current flow in a surrounded shielded cable. The shield current trap so described does not require a direct electrical connection to the shielded cable and so may float on the cable to be easily added, removed, or adjusted in position.
The effectiveness of this floating shield current trap requires that it be closely tuned to the expected frequency of the shield current. When such a trap is used with MRI equipment, the predominant shield currents will be equal to the Larmor frequency of precessing hydrogen protons within the magnetic field of the MRI machine.
The Larmor frequency depends on the strength of the magnet and varies among manufacturers for a given magnet size (e.g. 1.5 Tesla) and for different magnet sizes among a single manufacturer. Ideally, one such shield current trap could be used for all systems despite this variation in frequency.
The present invention provides an improved floating shield current trap that provides a simple tuning mechanism so that the trap may be used with different machines. Generally, the floating trap is divided along its axis into separate resonant loops. By adjusting the separation between these loops, the coupling between the loops is changed, adjusting the trap""s resonant frequency.
Specifically, the present invention provides a shield current trap having a first trap element with a first inner conductive channel joined at a respective first and second axial ends, via a first and second radial conductor, to corresponding first and second ends of a first outer conductive shell to form a first resonant loop. A second trap element having a second conductive channel joined at respective first and second axial ends, via third and fourth radial conductors, to first and second ends of a second outer conductive shell forms a second resonant loop. The first and second conductive channels may be assembled in opposition to enclose an axially extending shielded cable. A clamp assembly controls the separation of the first and second trap elements to control the coupling between the first and second resonant loops.
Thus, it is one object of the invention to provide a simple method of adjusting the frequency at which the trap is resonant and thereby accommodating both for manufacturing tolerances and variations between shield current frequencies in different applications.
At least one of the inner conductive channels, the outer conductive shell, and the first through fourth radial conductors include a series capacitor.
Thus, it is another object of the invention to allow capacitive tuning of the loops such as may provide more compact trap size.
The clamping means may be a spring clip fitting around the first and second trap elements to draw them together.
Another object of the invention is to provide a simple mechanical means for holding the trap elements together while allowing adjustment.
The clamping means may include adjustable standoffs extending from the first trap element to space the second trap element therefrom. In one embodiment, the standoffs may be set screws partially extending from threaded holes in the first trap element to extend outward therefrom to abut a portion of the second trap element.
Thus, it is another object of the invention to provide for easy adjustability of the shield trap without the need for a variety of shims or the like.
The clamping means may alternatively be a machine screw having a head engaging the first trap element and threads engaging a threaded hole in the second trap element to draw the first and second trap elements together with a tightening of the machine screw. The separation between the first and second trap elements may include a spring urging the first and second trap elements apart. That spring may be an elastomeric polymer.
Thus, it is another object of the invention to provide a simplified alternative adjustment mechanism.
The invention may include alignment guides holding the first and second trap elements in alignment for a range of separation of the first and second trap elements, for example, dowels and interfitting bores.
Thus, it is another object of the invention to hold the shells in alignment during the adjustment process simplifying the adjustment process and further preventing shifting after the adjustment is complete.
Another object of the invention is to provide an adjustment mechanism that allows separation of the trap for insertion of the shielded cable into the trap after the cable is connected to equipment or connectors such as would prevent the cable from being threaded through the bore of the trap.
These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention.